It has been known that a partially-saponified vinyl alcohol polymer (hereinafter, vinyl alcohol polymer is sometimes abbreviated as “PVA”) is used as a dispersion stabilizer for suspension polymerization of vinyl compounds (for example, vinyl chloride).
However, the use of a common partially-saponified PVA does not always satisfy the requirements; specifically, (1) even using a small amount, a high absorbency of a plasticizer can be achieved, resulting in easy processing, (2) the remaining monomer components can be easily removed, and (3) formation of coarse particles is reduced.
For achieving the above required properties, there have been proposed the use of, for example, a PVA with a low polymerization degree and a low saponification degree which has side chains having oxyalkylene groups, as a dispersion stabilizer for suspension polymerization for vinyl compounds (see Patent Reference Nos. 1 to 7), the use of a PVA having ionic groups (see Patent Reference Nos. 7 to 8), and the use of a PVA having alkyl groups with a saponification degree of more than 98 mol % (see Patent Reference No. 9). These various PVAs are highly water-soluble and easily handleable.
The above requirements (1) to (3) have, however, become stricter. Therefore, the methods using various PVAs as described in Patent Reference Nos. 1 to 9 have not been satisfactorily effective for achieving performance required at present. In particular, the requirement in terms of removal of monomer components described in (2) is very strict, for example, in production of a rigid polyvinyl chloride where polymerization is conducted at a temperature of 60° C. or higher.
In general, compared to a polyvinyl chloride produced by polymerization at a temperature lower than 60° C., a rigid polyvinyl chloride produced by polymerization at a temperature of 60° C. or higher has a problem that vinyl chloride monomers remaining in polyvinyl chloride particles cannot be easily removed in a drying process after polymerization so that prolonged drying under severe conditions is required for removing the remaining monomers. Therefore, there are very strong needs for saving in energy and time in the process for removing the remaining monomers. Thus, there is needed a dispersion stabilizer which allows a large amount of the remaining monomers to be removed in a shorter time. At present, there are no dispersion stabilizers which satisfy the requirements, including the various PVAs described in Patent Reference Nos. 1 to 9.